v1.13.1 — codegen: all methods per path + correct enum/null

Fixed

• codegen now emits every HTTP method on a shared path. Operations that
  reused an operationId across HTTP verbs — or used a verb-agnostic
  operationId — collapsed to a single generated name, so the typed client
  silently kept only the last method on a path (e.g. POST /users clobbered
  GET /users).
  Generated type and function names are now guaranteed unique; colliding names
  are disambiguated by HTTP method (usersGet / usersPost).

• codegen enum / nullable output is now correct. A null present both in
  a 3.1 type: ["string", "null"] array (or via nullable: true) and in an
  enum produced a doubled union such as "active" | "inactive" | null | null.
  Nullability is now folded into a single trailing | null and enum members are
  deduplicated. Type coverage was extended along the way to const literals,
  prefixItems tuples, and closed objects (additionalProperties: false →
  Record<string, never>).